This invention relates to a layer of a organic-metallic complex of a polyamine with a metal oxide, or metallate ion, or water-soluble metallate salt for reducing the rate of corrosion of metal surfaces in aqueous systems having a pH greater than about 7. Such a layer is formed on the surfaces of steel, iron and aluminum subject to corrosion by process water recirculated through cooling towers, cooling water for internal combustion engines which water is recirculated through heat exchange means, water in industrial boilers for the production of process steam, and comparable aqueous systems. This layer is also formed on ferrous and aluminum surfaces, and surfaces of alloys thereof, when the complex is in an aqueous synthetic metal-working fluid, or, in a water-based adhesive used on a metal surface, or, in an aqueous oi-field drilling mud in contact with steel drilling equipment, or, in an aqueous rinse for a bare, that is, untreated steel surface, or a treated, for example phosphated steel surface.
This invention more specifically relates to an amine-metallic layer formed in weakly acidic, neutral or alkaline aqueous systems by the chemical combination of a substantially linear polyamine, or a branched polyamine, either of which necessarily contains at least four (4) amine groups, two of which are secondary amine groups, with an oxide or metallate ion of molybdenum (Mo) or tungsten (W), so that when the primary amine group is anchored directly to the metal surface, the effect of the N atoms stretched over the length of the polyamine produces a "caging effect" which immobilizes the oxide or metallate ions thus forming the protective layer.
The corrosion inhibiting amine-metallic layer is thus a combination of a particular polyamine (referred to herein as "the polyamine component") and an oxide or metallate ionn of Mo or W (either of which is referred to herein as "the metal component"). The precise structure of the layer depends upon the pH of the aqueous system containing the components, inter alia. We have found that, as the pH increases above 7, the rate of formation of an amine metal oxide layer, which is a predominantly coordination complex of polyamine and metal oxide having the characteristics of a coordination increases. As the pH decreases, ion pair interaction between protonated amine and metallate anions predominates, and such ion pair interaction decreases as the pH increases, resulting in the amine-metallate complex having the characteristics of an ionic complex. The term "amine-metallate complex" is used herein to connote a solid ionic complex, or an admixture which, in solution, behaves like the ionic complex.
The term "polyamine" is used herein to specify an essentially linear oligomer free from tertiary amine groups, for example derived from ethylene diamine (EDA) or ethyleneimine (EI) monomer; and, also to specify branched oligomers and polymers containing tertiary amine groups, commercially derived by oligomerization or polymerization of EI. The term "oligomer" is used when the polyamine formed has relatively few repeating units, generally less than 10.
The prior art is replete with a wide array of additives for use in aqueous systems where corrosion of metal equipment and scale formation is a problem. These additives, for the purpose of inhibiting corrosion of metal surfaces and suppressing scale formation thereon, include inorganic salts, such as the chromate, phosphate, nitrite and molybdate salts, organic compounds such as various acyclic alkylamines, cycloalkylamines, heterocyclic amines, etc., but the use of organic-metallic complexes or coordination complexes such as zinc tannin, is relatively rare. Recently chromate salts have fallen into disfavor and have been replaced with sodium molybdate due mainly to the toxicity of the chromates, the level of which is regulated by Federal legislation. Hence, the emphasis on organic compounds such as the acrylic polymers, fatty acid sulfonates, and 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP).
It is well known that the molecular structure of organic molecules influences the adsorbability, and correspondingly, the effectiveness of corrosion inhibitors. The general aspects of this topic are set forth in several texts, for example Corrosion Control By Coatings, edited by H. Leidheiser, Jr. (Science Press, Princeton, 1979). Despite proposed quantitative relationships between inhibitor effectiveness and specific characteristics attributable to the inhibitor, for example the Hammet constant which is a measure of a group's ability to modify electron configuration, or, to other characteristics such as the ability to change the spatial configuration of the groups, we know of no way to predict the efficacy of a proposed inhibitor with substantial quantitative accuracy.
An immobilized EDA group, attached through silicon attachment points, to a silica gel substrate, or to silylated controlled pore glass (CPG) beads, retained molybdate or tungstate which is monomeric (see "Structural Studies of Immobilied Ethylenediamine as a Preconcentrating Agent for Molybdate and Tungstate" by Leyden, D. E., et al, Analytica Chemica Acta, 100 (1978) 545-554), but when a precipitate was formed in EDA, the precipitate contained polymeric molybdate species. Study of this phenomenon was extended to alkylaminosilanes and aminosilanecarboxylates immobilzed on a silica substrate. But alkylaminosilanes and aminosilanecarboxylates, whether diamines or triamines, form complexes which in general are easily hydrolyzed and are unsuitable for use as corrosion inhibitors, as are complexes of amines which are not organic polyamines.
By "organic polyamines" we refer to amines which do not have a silicon, phosphorus, boron or other metal or metal-like elemental component. In particular, amines in which the Si is replaced with P or B are also unsuitable as corrosion inhibitors. Such unsuitability may be attributable to the aforementioned amines in these prior art complexes being doubly protonated in acidic solutions, indicating that an ion pair is formed, giving these complexes the characteristics of ionic compounds.
Moreover, the determination by Leydon et al that the stability of the adsorption bonding to the metal determines the fixation effectiveness, was observed only in an acid aqueous system and only where the amine was attached to a silica gel or CPG bead through a silicon attachment point. Most significant is that the EDA and triethylenetetramine (TETA), modified with silane groups to present Si attachment points, were "too short" chain length), so termed because they had too few amine groups to produce a caging effect sufficient to hold and immobilize the molybdate ion, though the carbon spacing (number of carbon atoms between amine N atoms) was satisfactory.
The failure to recognize the criticality of having at least 4 amine groups in a polyamine chain to provide excellent immobilization of molybdate ions apparently stemmed from an earlier study which showed that the effectiveness of Dow Corning Z-6020 (which has only two primary amine functional groups) for the extraction of MoO.sub.4.sup.= (molybdate ion) dropped quickly as the pH rose from 3 to about 7; and, the extraction of WO.sub.4.sup.= (tungstate ion) dropped even more quickly as the pH rose from about 5 until there was no extraction of tungstate ion at about pH 10 (see "Preconcentration of Certain Anions Using Reagents Immobilized via Silylation" by Leyden, D. E. et al, Anal. Chem. 48, No. 1, 67-70, Jan 1976). Though the formation of an immobilized layer of silylated amine was established by ESCA techniques, it is clear that the extraction of MoO.sub.4.sup.= and WO.sub.4 = with the too-short silylated amines was specifically contraindicated in neutral or alkaline aqueous systems.
Among the organic polyamines, it is known that polymeric amines having only a very few repeating units in a polymer chain are more efficient corrosion inhibitors in acidic aqueous solutions than the corresponding monomers because of multiple absorption bonding. For example, poly(ethyleneamine) formed from EDA monomer, in particular, tetraethylenepentamine (TEPA) and TETA gave higher percentage inhibition than EDA, as did hexamethylenediamine (HMDA) which is not a polyamine but a diamine. (See "Inhibition of Acid Corrosion by Soluble Monomer and Polymer Amines Containing Identical Functional Groups" by Annand, R. R. et al., Jour. Electrochem. Soc., Feb. 1965, 144-148). However, under neutral or alkaline conditions we found the polyamine by themselves to be much less effective. Under the particular acidic conditions of Annand et al WO.sub.3 does not go into solution, and MoO.sub.3 forms oxyhalides.
By a "polyamine" which is effective in our invention, we specifically refer to an amine having an amine functionality of at least four (4), that is, having at least 4 amine groups, at least one of which is primary, the remaining being either secondary or tertiary. The complexes which form the layer in neutral or alkaline aqueous systems undergo a change from a fluent condition to a non-fluent or immobilized layer on a steel, aluminum or other metal surface to form a complex which may be formed as (i) a coordination complex which has the characteristics of a coordination compound, or (ii) an interaction of ion pairs having the characteristics of an ionic complex, or (iii) an admixture of water-soluble metallate salts and the polyamine, which admixture, for the purposes of this invention, will be referred to as a complex, or `admixture complex`, because the admixture behaves as a complex does. The complex and the layer formed by it, have distinctly different and unexpectedly more favorable characteristics of corrosion inhibition than either a polyamine by itself, or the metal component by itself, because neither the polyamine nor the metal component by itself forms the necessary layer of organic-metallic complex.
In the prior art, amine molybdates have been precipitated from an aqueous solution of an amine and MoO.sub.3, molybdic acid or a molybdenum salt. To ensure precipitation of the desired solid amine molybdate, when a molybdenum salt was used, the reaction was carried out in the presence of a suitable acid. The amount of each component used was stoichiometric, or with an excess of Mo compound, so that the solid, a useful fire retardant, would precipitate from solution (see U.S. Pat. No. 4,161,466 to Kroenke, W. J.). The solid obtained was identified as an ionic complex, leaving an acidic solution (having a pH less than 6) of the byproduct salt of the acid used. An analogous reaction between an amine and MoO.sub.3 was carried out in the presence of a supporting electrolyte, for example ammonium chloride, in U.S. Pat. No. 4,217,292 to Kroenke, W. J. Again the reaction produced a solid ionic complex, leaving an acidic solution of the supporting electrolyte, in this case, ammonium chloride.
In the case of an amine, a Mo salt and an acid, there is nothing to indicate that a coordination complex, or an ionic complex, or an admixture complex might form as a stable solution. Similarly, in the case of an amine and MoO.sub.3 there is nothing to indicate that a coordination complex, or an ionic complex might form as a stable solution. In either case, there is especially no reason to expect that any complex would form as a stable solution under neutral or basic conditions, in a contraction less than about 5% by wt.